Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
Optical encoders utilize a light source and a photo detector to measure changes in the position of an encoding disk or strip. In a transmissive encoder, the encoding disk includes a series of alternating opaque and transparent stripes. The light source is located on one side of the code strip, and the photodetector is located on the other side of the code strip. The light source and photodetector are fixed relative to one another, and the code strip moves between the light source and the photodetector such that the light reaching the photodetector is interrupted by the opaque regions of the code strip. The position of the code strip is determined by measuring the transitions between the light and dark regions observed by the photodiode.
In a reflective encoder, the light source and photodetector are located on the same side of the code strip, and the code strip consists of alternating reflective and absorbing stripes or bands. The light source and the associated optical system are positioned such that light from the light source is imaged into the detector when the light is reflected from the reflective strips.
Transmissive encoders have a number of advantages over reflective encoders in terms of tolerance and contrast ratios. In a reflective encoder, the distance between the code strip and the detector is critical as either the code strip itself or the light source as seen in the reflected light from the code strip is imaged onto the detector. Hence, if there is an error in the code strip to detector distance, the image will be out of focus and errors will result.
In a transmissive encoder, the light from the light source is colliminated before it reaches the code strip, and hence, the light leaving the code strip is also colliminated. The detection assembly needs only to image this colliminated light onto the detector surface. Hence, the only critical distance is the distance from the imaging lens to the detector, which can be tightly controlled by the detector manufacturer independent of the specific encoder assembly.
In addition, the code strips for reflective encoders have a contrast ratio determined by the ratio of the reflectance of the reflective and absorptive regions. This ratio tends to be less than the ratio of the absorbance of the clear and opaque regions of a transmissive code strip.
Unfortunately, transmissive recorders require that two separate components, the light source and the photodetector, be mounted and aligned with one another at the time of assembly of the encoder. Reflective encoders, in contrast, are constructed from a single emitter-detector element that is packaged together with the various optical components for imaging the light source onto the photodetector. This reduces the cost of assembly.
While reflective encoders provide advantages in terms of requiring only a single emitter detector module, a different emitter detector module is often required when the size or resolution of the code strip is altered. Hence, a number of different emitter detector modules must be stocked to provide emitter detector modules for the various possible code strip designs.